Moving handrails on an escalator or moving walkway are typically driven by passing the handrails through a driving pressure nip along the return path of travel of the handrail beneath the balustrades. The nip may be formed by a pair of cooperating rollers, or by a driven belt which cooperates with a plurality of backup rollers. The nip will be powered by chains or the like which are driven by the main drive mechanism of the escalator. Soviet Patent No. SU1286-493A, U.S. Pat. No. 4,134,883, and Austrian Patent No. 247,236 disclose variations of the prior art drive systems described above.
U.S. Pat. No. 5,117,960, granted Jun. 2, 1992 to H. W. Ahls, et al. discloses a handrail drive system which uses a powered drive belt and a pressure belt to drive the handrail along its path of travel. The drive belt is entrained on a powered drive roller, and a free wheeling idler roller. The idler roller is biased by a spring to provide an adjustable tension to the drive belt. A series of adjustable but non-biased backup rollers provide a backing force for the drive belt which holds the latter against the handrail between the drive roller and the idler roller. The pressure belt is entrained on a pair of idler rollers, one of which is spring biased to provide pressure belt tension. A plurality of pressure rollers are disposed between the pressure idler rollers and are individually spring biased against the pressure belt so as to press the latter against the handrail.
The handrail drive system described in the aforesaid U.S. Pat. No. 5,117,960 patent is serviceable, but exhibits certain drawbacks. The use of a pressure belt requires additional hardware to mount the pressure belt and does not add any drive power or stability to the system. The use of individual pressure roller springs renders the drive assembly difficult to properly adjust. The individual pressure springs also limit the flexibility of the force imparted to the handrail which presses the handrail against the drive belt. Finally, the tension spring assemblies which are used to impart tension to the drive belt and the pressure belt, and thus reduce or eliminate belt slippage, are vulnerable to forces which emanate from the handrail that tend to vary the belt tension depending on whether the handrail is being moved in the upward or downward direction, i.e., toward, or away from the belts' tension rollers.
When the handrail is moved in the upward direction, there is greater frictional drag imparted to the handrail by the guide rails which must be overcome by the drive assembly, than when the handrail is moved in the downward direction. When the handrail drive assembly is installed on the escalator, the belt power roller will be below the belt tension roller, and that relationship will not change, whether the handrail is being moved in the upward or downward direction. Thus, the handrail will be moved toward the tension roller when the handrail moves in the upward direction and away from the tension roller when the handrail moves in the downward direction. Since the tension roller is always biased away from the drive roller, the direction of movement of the handrail will tend to lessen the degree of compression of the tension spring if the drive belt is moving toward the tension roller; and will tend to increase the degree of compression of the tension spring if the drive belt is moving away from the tension roller. When the tension spring is further compressed, a decrease in drive belt tension ensues with a concurrent lessening of the driving force applied to the handrail and even drive belt slippage. The result of the aforesaid drive belt tension instability is an inability to accurately control the bi-directional drive force imposed on the handrail by the drive belt. Drive belt tension must be adjusted to take into account the desired direction of movement of the handrail. This factor mitigates against the use of escalators that can be directionally reversed to account for passenger traffic flow. The same applies to horizontal moving walkways, which are typically much longer than escalators.